In an existing frequency division duplex (FDD) radio communications system, a mechanism is required to support the FDD system in obtaining symmetry of uplink and downlink channels. At present, a unified division duplex (UDD) mode is used to solve the problem. The practice is to use a frame format shown in FIG. 1, where fUL is an uplink carrier frequency of the FDD system and fDL is a downlink carrier frequency of the FDD system. Each subframe represents a transmission time window. In the FDD system, subframes with the same number on different carrier frequencies may be transmitted simultaneously. For example, subframe 0 on the uplink carrier frequency and subframe 0 on the downlink carrier frequency may be transmitted simultaneously. In the frame structure, subframe 2 and subframe 7 on the downlink carrier frequency are special subframes. A special subframe is decomposed into three timeslots: a downlink (DL) timeslot for downlink transmission, a guard period (GP), and an uplink (UL) timeslot for uplink transmission, which are represented by DL, GP, and UL respectively in the accompanying drawings. A sounding reference symbol (SRS) may be transmitted in a DL timeslot, where the SRS is used to reflect a downlink channel state, so that a NodeB can obtain downlink channel state information by using the symmetry of uplink and downlink channels.